Lithium metal secondary battery containing an elastic anode-protecting layer

ABSTRACT

Provided is a lithium metal secondary battery comprising a cathode, an anode, and a non-solid state electrolyte without a porous separator disposed between the cathode and the anode, wherein the anode comprises: (a) an anode active material layer containing a layer of lithium or lithium alloy, in a form of a foil, coating, or multiple particles aggregated together, as an anode active material; and (b) an anode-protecting layer in physical contact with the anode active material layer, having a thickness from 1 nm to 100 μm and comprising an elastomer having a fully recoverable tensile elastic strain from 2% to 1,000% and a lithium ion conductivity from 10 −8  S/cm to 5×10 −2  S/cm when measure at room temperature; wherein the lithium metal secondary battery does not include a lithium-sulfur battery or a lithium-selenium battery.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 16/014,614, filed Jun. 21, 2018, which is herebyincorporated by reference for all purposes.

FIELD OF THE INVENTION

The present invention relates to the field of rechargeable lithium metalbattery having a lithium metal layer (in a form of thin lithium foil,coating, or sheet of lithium particles) as an anode active material anda method of manufacturing same.

BACKGROUND OF THE INVENTION

Lithium-ion and lithium (Li) metal cells (including lithium metalsecondary cell, lithium-sulfur cell, lithium-selenium cell, Li-air cell,etc.) are considered promising power sources for electric vehicle (EV),hybrid electric vehicle (HEV), and portable electronic devices, such aslap-top computers and mobile phones. Lithium metal has the highestcapacity (3,861 mAh/g) compared to any other metal or metal-intercalatedcompound (except Li_(4.4)Si) as an anode active material. Hence, ingeneral, rechargeable Li metal batteries have a significantly higherenergy density than lithium ion batteries.

Historically, rechargeable lithium metal batteries were produced usingnon-lithiated compounds having high specific capacities, such as TiS₂,MoS₂, MnO₂, CoO₂ and V₂O₅, as the cathode active materials, which werecoupled with a lithium metal anode. When the battery was discharged,lithium ions were dissolved from the lithium metal anode and transferredto the cathode through the electrolyte and, thus, the cathode becamelithiated. Unfortunately, upon cycling, the lithium metal resulted inthe formation of dendrites that ultimately caused unsafe conditions inthe battery. As a result, the production of these types of secondarybatteries was stopped in the early 1990's giving ways to lithium-ionbatteries.

Even now, cycling stability and safety concerns remain the primaryfactors preventing the further commercialization of Li metal batteriesfor EV, HEV, and microelectronic device applications. These issues areprimarily due to the high tendency for Li to form dendrite structuresduring repeated charge-discharge cycles or an overcharge, leading tointernal electrical shorting and thermal runaway. Many attempts havebeen made to address the dendrite-related issues, as briefly summarizedbelow:

Fauteux, et al. [D. Fauteux, et al., “Secondary Electrolytic Cell andElectrolytic Process,” U.S. Pat. No. 5,434,021, Jul. 18, 1995] appliedto a metal anode a protective surface layer (e.g., a mixture ofpolynuclear aromatic and polyethylene oxide) that enables transfer ofmetal ions from the metal anode to the electrolyte and back. The surfacelayer is also electronically conductive so that the ions will beuniformly attracted back onto the metal anode during electrodeposition(i.e. during battery recharge). Alamgir, et al. [M. Alamgir, et al.“Solid polymer electrolyte batteries containing metallocenes,” U.S. Pat.No. 5,536,599, Jul. 16, 1996] used ferrocenes to prevent chemicalovercharge and dendrite formation in a solid polymer electrolyte-basedrechargeable battery.

Skotheim [T. A. Skotheim, “Stabilized Anode for Lithium-PolymerBattery,” U.S. Pat. No. 5,648,187 (Jul. 15, 1997); U.S. Pat. No.5,961,672 (Oct. 5, 1999)] provided a Li metal anode that was stabilizedagainst the dendrite formation by the use of a vacuum-evaporated thinfilm of a Li ion-conducting polymer interposed between the Li metalanode and the electrolyte. Skotheim, et al. [T. A. Skotheim, et al.“Lithium Anodes for Electrochemical Cells,” U.S. Pat. No. 6,733,924 (May11, 2004); U.S. Pat. No. 6,797,428 (Sep. 28, 2004); U.S. Pat. No.6,936,381 (Aug. 30, 2005); and U.S. Pat. No. 7,247,408 (Jul. 24, 2007)]further proposed a multilayer anode structure consisting of a Limetal-based first layer, a second layer of a temporary protective metal(e.g., Cu, Mg, and Al), and a third layer that is composed of at leastone layer (typically 2 or more layers) of a single ion-conducting glass,such as lithium silicate and lithium phosphate, or polymer. It is clearthat such an anode structure, consisting of at least 3 or 4 layers, istoo complex and too costly to make and use.

Protective coatings for Li anodes, such as glassy surface layers ofLiI—Li₃PO₄—P₂S₅, may be obtained from plasma assisted deposition [S. J.Visco, et al., “Protective Coatings for Negative Electrodes,” U.S. Pat.No. 6,025,094 (Feb. 15, 2000)]. Complex, multi-layer protective coatingswere also proposed by Visco, et al. [S. J. Visco, et al., “ProtectedActive Metal Electrode and Battery Cell Structures with Non-aqueousInterlayer Architecture,” U.S. Pat. No. 7,282,295 (Oct. 16, 2007); U.S.Pat. No. 7,282,296 (Oct. 16, 2007); and U.S. Pat. No. 7,282,302 (Oct.16, 2007)].

Despite these earlier efforts, no rechargeable Li metal batteries haveyet succeeded in the market place. This is likely due to the notion thatthese prior art approaches still have major deficiencies. For instance,in several cases, the anode or electrolyte structures are too complex.In others, the materials are too costly or the processes for makingthese materials are too laborious or difficult. Solid electrolytestypically have a low lithium ion conductivity, are difficult to produceand difficult to implement into a battery.

Furthermore, solid electrolyte, as the sole electrolyte in a cell or asan anode-protecting layer (interposed between the lithium film and theliquid electrolyte) does not have and cannot maintain a good contactwith the lithium metal. This effectively reduces the effectiveness ofthe electrolyte to support dissolution of lithium ions (during batterydischarge), transport lithium ions, and allowing the lithium ions tore-deposit back onto the lithium anode (during battery recharge).

Another major issue associated with the lithium metal anode is thecontinuing reactions between electrolyte and lithium metal, leading torepeated formation of “dead lithium-containing species” that cannot bere-deposited back to the anode and become isolated from the anode. Thesereactions continue to irreversibly consume electrolyte and lithiummetal, resulting in rapid capacity decay. In order to compensate forthis continuing loss of lithium metal, an excessive amount of lithiummetal (3-5 times higher amount than what would be required) is typicallyimplemented at the anode when the battery is made. This adds not onlycosts but also a significant weight and volume to a battery, reducingthe energy density of the battery cell. This important issue has beenlargely ignored and there has been no plausible solution to this problemin battery industry.

Clearly, an urgent need exists for a simpler, more cost-effective, andeasier-to-implement approach to preventing Li metal dendrite-inducedinternal short circuit and thermal runaway problems in Li metalbatteries, and to reducing or eliminating the detrimental reactionsbetween lithium metal and the electrolyte.

Hence, an object of the present invention was to provide an effectiveway to overcome the lithium metal dendrite and reaction problems in alltypes of Li metal batteries having a lithium metal anode. A specificobject of the present invention was to provide a lithium metal cell thatexhibits a high specific capacity, high specific energy, high degree ofsafety, and a long and stable cycle life.

SUMMARY OF THE INVENTION

Herein reported is a lithium metal secondary battery comprising acathode (positive electrode), an anode (negative electrode), and anon-solid state electrolyte without a porous separator disposed betweenthe cathode and the anode, wherein the anode comprises: (a) an anodeactive material layer containing a layer of lithium or lithium alloy, ina form of a foil, coating, or multiple particles aggregated together, asan anode active material; and (b) an anode-protecting layer in physicalcontact with the anode active material layer, having a thickness from 1nm to 100 μm and comprising an elastomer having a fully recoverabletensile elastic strain from 2% to 1,000% and a lithium ion conductivityfrom 10⁻⁸ S/cm to 5×10⁻² S/cm when measure at room temperature; whereinthe lithium metal secondary battery does not include a lithium-sulfurbattery or lithium-selenium battery.

Preferably, the anode active material layer, the elastomer-basedanode-protecting layer, and the cathode layer are laminated together insuch a manner (e.g. roll-pressed together) that the resulting cell isunder a compressive stress or strain for the purpose of maintaining agood contact between the anode active material layer and theanode-protecting layer.

In the lithium metal secondary battery, the non-solid state electrolyteis selected from organic liquid electrolyte, ionic liquid electrolyte,polymer gel electrolyte, quasi-solid electrolyte having a lithium saltdissolved in an organic or ionic liquid with a lithium saltconcentration higher than 2.0 M (from 2.0M to 14 M; typically from 2.5 Mto 10 M; and more typically from 3.5M to 7 M), or a combination thereof.

It is well-known in the art that a porous separator may not be necessaryif the electrolyte is a solid-state electrolyte; but, a porous separatoris normally required in order to electronically separate the anode fromthe cathode if the electrolyte contains a liquid ingredient, such as inan organic liquid electrolyte, ionic liquid electrolyte, polymer gelelectrolyte (polymer+liquid solvent), and quasi-solid electrolyte. Theelastomer-based anode-protecting layer itself acts as a separator toelectrically isolate the anode and the cathode. This protective layer,being as thin as a few nanometers and typically from 10 nm to 10 μm, issignificantly thinner than the typically >20 μm in thickness of theconventional porous separator. Yet, this elastomer also plays the rolesof protecting the lithium anode, preventing lithium dendrite formationand penetration, provides an environment conducive to uniform anduninterrupted transport and re-deposition of lithium ions, etc. Thereduced weight and volume also leads to increased specific energy(Wh/kg) and volumetric energy density (Wh/L).

The foil or coating of lithium or lithium alloy may be supported by acurrent collector (e.g. a Cu foil, a Ni foam, a porous layer ofnanofilaments, such as graphene sheets, carbon nanofibers, carbonnanotubes, etc.).

For defining the claims, the invented lithium metal secondary batterydoes not include a lithium-sulfur cell or lithium-selenium cell. Assuch, the cathode does not include sulfur, lithium polysulfide,selenium, and lithium polyselenide.

The elastomer (sulfonated or non-sulfonated) is a high-elasticitymaterial which exhibits an elastic deformation that is at least 2%(preferably at least 5% and up to approximately 1,000%) when measuredunder uniaxial tension. In the field of materials science andengineering, the “elastic deformation” is defined as a deformation of amaterial (when being mechanically stressed) that is essentially fullyrecoverable upon release of the load and the recovery process isessentially instantaneous (no or little time delay). The elasticdeformation is more preferably greater than 10%, even more preferablygreater than 30%, further more preferably greater than 50%, and stillmore preferably greater than 100%.

In some embodiments, the elastomer preferably and more typically has afully recoverable elastic tensile strain from 5% to 300% (most typicallyfrom 10% to 150%), a thickness from 10 nm to 20 μm, and an electricalconductivity of at least 10⁻⁴ S/cm when measured at room temperature ona cast thin film 20 μm thick.

Preferably, the elastomer contains a sulfonated or non-sulfonatedversion of natural polyisoprene, synthetic polyisoprene, polybutadiene,chloroprene rubber, polychloroprene, butyl rubber, styrene-butadienerubber, nitrile rubber, ethylene propylene rubber, ethylene propylenediene rubber, metallocene-based poly(ethylene-co-octene) (POE)elastomer, poly(ethylene-co-butene) (PBE) elastomer,styrene-ethylene-butadiene-styrene (SEBS) elastomer, epichlorohydrinrubber, polyacrylic rubber, silicone rubber, fluorosilicone rubber,perfluoroelastomers, polyether block amides, chlorosulfonatedpolyethylene, ethylene-vinyl acetate, thermoplastic elastomer, proteinresilin, protein elastin, ethylene oxide-epichlorohydrin copolymer,polyurethane, urethane-urea copolymer, or a combination thereof. Theseelastomers or rubbers, when present without graphene sheets, exhibit ahigh elasticity (having a fully recoverable tensile strain from 2% to1,000%). In other words, they can be stretched up to 1,000% (10 times ofthe original length when under tension) and, upon release of the tensilestress, they can fully recover back to the original dimension. By addingfrom 0.01% to 50% by weight of a conductive reinforcement materialand/or a lithium ion-conducting species dispersed in an elastomericmatrix material, the fully recoverable tensile strains are typicallyreduced down to 2%-500% (more typically from 5% to 300% and mosttypically from 10% to 150%).

The elastomer, if sulfonated, becomes significantly more lithiumion-conducting. The lithium ion conductivity of an elastomer, sulfonatedor un-sulfonated, may be further improved if some desired amount oflithium ion-conducting additive is incorporated into the elastomermatrix.

It may be noted that lithium foil/coating layer may decrease inthickness due to dissolution of lithium into the electrolyte to becomelithium ions as the lithium battery is discharged, creating a gapbetween the current collector and the protective layer if the protectivelayer were not elastic. Such a gap would make the re-deposition oflithium ions back to the anode impossible. We have observed that theinstant elastomer layer is capable of expanding or shrinking congruentlyor conformably with the anode layer. This capability helps to maintain agood contact between the current collector (or the lithium film itself)and the protective layer, enabling the re-deposition of lithium ionswithout interruption.

The elastomer may further contain a lithium salt selected from lithiumperchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithiumborofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-methanesulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimidelithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃),Li-Fluoroalkyl-Phosphates (LiPF₃(CF₂CF₃)₃), lithiumbisperfluoro-ethysulfonylimide (LiBETI), lithiumbis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide,lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-basedlithium salt, or a combination thereof.

At the anode side, preferably and typically, the elastomer in theprotective layer is designed or selected to have a lithium ionconductivity no less than 10⁻⁵ S/cm, more preferably no less than 10⁻⁴S/cm, and most preferably no less than 10⁻³ S/cm. Some of the selectedelastomers, when sulfonated, can exhibit a lithium-ion conductivitygreater than 10⁻² S/cm. In some embodiments, the elastomer is anelastomer containing no additive or filler dispersed therein. In others,the elastomer composite is an elastomer matrix composite containing from0.1% to 40% by weight (preferably from 1% to 30% by weight) of a lithiumion-conducting additive dispersed in an elastomer matrix material.

In some embodiments, the elastomer is selected from a sulfonated orun-sulfonated version of natural polyisoprene (e.g. cis-1,4-polyisoprenenatural rubber (NR) and trans-1,4-polyisoprene gutta-percha), syntheticpolyisoprene (IR for isoprene rubber), polybutadiene (BR for butadienerubber), chloroprene rubber (CR), polychloroprene (e.g. Neoprene,Baypren etc.), butyl rubber (copolymer of isobutylene and isoprene,IIR), including halogenated butyl rubbers (chloro butyl rubber (CIIR)and bromo butyl rubber (BIIR), styrene-butadiene rubber (copolymer ofstyrene and butadiene, SBR), nitrile rubber (copolymer of butadiene andacrylonitrile, NBR), EPM (ethylene propylene rubber, a copolymer ofethylene and propylene), EPDM rubber (ethylene propylene diene rubber, aterpolymer of ethylene, propylene and a diene-component),epichlorohydrin rubber (ECO), polyacrylic rubber (ACM, ABR), siliconerubber (SI, Q, VMQ), fluorosilicone rubber (FVMQ), fluoroelastomers(FKM, and FEPM; such as Viton, Tecnoflon, Fluorel, Aflas and Dai-El),perfluoroelastomers (FFKM: Tecnoflon PFR, Kalrez, Chemraz, Perlast),polyether block amides (PEBA), chlorosulfonated polyethylene (CSM; e.g.Hypalon), and ethylene-vinyl acetate (EVA), thermoplastic elastomers(TPE), protein resilin, protein elastin, ethylene oxide-epichlorohydrincopolymer, polyurethane, urethane-urea copolymer, or a combinationthereof.

In some embodiments, the elastomer further contains a lithiumion-conducting additive dispersed therein, wherein the lithiumion-conducting additive is selected from Li₂CO₃, Li₂O, Li₂C₂O₄, LiOH,LiX, ROCO₂Li, HCOLi, ROLi, (ROCO₂Li)₂, (CH₂OCO₂Li)₂, Li₂S, Li_(x)SO_(y),or a combination thereof, wherein X═F, Cl, I, or Br, R=a hydrocarbongroup, 0<x≤1 and 1≤y≤4.

In some embodiments, the elastomer may form a mixture, blend, orsemi-IPN with a lithium ion-conducting polymer selected frompoly(ethylene oxide) (PEO), polypropylene oxide (PPO),poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA),poly(vinylidene fluoride) (PVDF), poly bis-methoxyethoxyethoxide-phosphazene, polyvinyl chloride, polydimethylsiloxane,poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a sulfonatedderivative thereof, or a combination thereof. Sulfonation is hereinfound to impart improved lithium ion conductivity to a polymer.

In certain embodiments, the elastomer comprises from 0.01% to 50% of anelectrically non-conducting reinforcement material dispersed therein,wherein the reinforcement material is selected from a glass fiber,ceramic fiber, polymer fiber, or a combination thereof. The electricallynon-conductive reinforcement may also be selected from glass particles,ceramic particles, polymer particles, etc. The reinforcement materialcan increase the mechanical strength and the lithium dendritepenetration resistance of the elastomer layer.

The cathode active material may be selected from an inorganic material,an organic material, a polymeric material, or a combination thereof. Theinorganic material may be selected from a metal oxide, metal phosphate,metal silicide, metal selenide, metal sulfide, or a combination thereof.

The inorganic material may be selected from a lithium cobalt oxide,lithium nickel oxide, lithium manganese oxide, lithium vanadium oxide,lithium-mixed metal oxide, lithium iron phosphate, lithium manganesephosphate, lithium vanadium phosphate, lithium mixed metal phosphate,lithium metal silicide, or a combination thereof.

In certain preferred embodiments, the inorganic material is selectedfrom a metal fluoride or metal chloride including the group consistingof CoF₃, MnF₃, FeF₃, VF₃, VOF₃, TiF₃, BiF₃, NiF₂, FeF₂, CuF₂, CuF, SnF₂,AgF, CuCl₂, FeCl₃, MnCl₂, and combinations thereof. In certain preferredembodiments, the inorganic material is selected from a lithiumtransition metal silicate, denoted as Li₂MSiO₄ or Li₂Ma_(x)Mb_(y)SiO₄,wherein M and Ma are selected from Fe, Mn, Co, Ni, V, or VO; Mb isselected from Fe, Mn, Co, Ni, V, Ti, Al, B, Sn, or Bi; and x+y≤1.

In certain preferred embodiments, the inorganic material is selectedfrom a transition metal dichalcogenide, a transition metaltrichalcogenide, or a combination thereof. The inorganic material isselected from TiS₂, TaS₂, MoS₂, NbSe₃, MnO₂, CoO₂, an iron oxide, avanadium oxide, or a combination thereof.

The cathode active material layer may contain a metal oxide containingvanadium oxide selected from the group consisting of VO₂, Li_(x)VO₂,V₂O₅, Li_(x)V₂O₅, V₃O₈, Li_(x)V₃O₈, Li_(x)V₃O₇, V₄O₉, Li_(x)V₄O₉, V₆O₁₃,Li_(x)V₆O₁₃, their doped versions, their derivatives, and combinationsthereof, wherein 0.1<x<5.

The cathode active material layer may contain a metal oxide or metalphosphate, selected from a layered compound LiMO₂, spinel compoundLiM₂O₄, olivine compound LiMPO₄, silicate compound Li₂MSiO₄, Tavoritecompound LiMPO₄F, borate compound LiMBO₃, or a combination thereof,wherein M is a transition metal or a mixture of multiple transitionmetals.

In some embodiments, the inorganic material is selected from: (a)bismuth selenide or bismuth telluride, (b) transition metaldichalcogenide or trichalcogenide, (c) sulfide, selenide, or tellurideof niobium, zirconium, molybdenum, hafnium, tantalum, tungsten,titanium, cobalt, manganese, iron, nickel, or a transition metal; (d)boron nitride, or (e) a combination thereof.

The cathode active material layer may contain an organic material orpolymeric material selected from poly(anthraquinonyl sulfide) (PAQS), alithium oxocarbon, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA),poly(anthraquinonyl sulfide), pyrene-4,5,9,10-tetraone (PYT),polymer-bound PYT, quino(triazene), redox-active organic material,tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE),2,3,6,7,10,11-hexamethoxytriphenylene (HMTP), poly(5-amino-1,4-dyhydroxyanthraquinone) (PADAQ), phosphazene disulfide polymer ([(NPS₂)₃]n),lithiated 1,4,5,8-naphthalenetetraol formaldehyde polymer,hexaazatrinaphtylene (HATN), hexaazatriphenylene hexacarbonitrile(HAT(CN)₆), 5-benzylidene hydantoin, isatine lithium salt, pyromelliticdiimide lithium salt, tetrahydroxy-p-benzoquinone derivatives (THQLi₄),N,N′-diphenyl-2,3,5,6-tetraketopiperazine (PHP),N,N′-diallyl-2,3,5,6-tetraketopiperazine (AP),N,N′-dipropyl-2,3,5,6-tetraketopiperazine (PRP), a thioether polymer, aquinone compound, 1,4-benzoquinone, 5,7,12,14-pentacenetetrone (PT),5-amino-2,3-dihydro-1,4-dyhydroxy anthraquinone (ADDAQ),5-amino-1,4-dyhydroxy anthraquinone (ADAQ), calixquinone, Li₄C₆O₆,Li₂C₆O₆, Li₆C₆O₆, or a combination thereof.

The thioether polymer is selected frompoly[methanetetryl-tetra(thiomethylene)] (PMTTM),poly(2,4-dithiopentanylene) (PDTP), a polymer containingpoly(ethene-1,1,2,2-tetrathiol) (PETT) as a main-chain thioetherpolymers, a side-chain thioether polymer having a main-chain consistingof conjugating aromatic moieties, and having a thioether side chain as apendant, poly(2-phenyl-1,3-dithiolane) (PPDT),poly(1,4-di(1,3-dithiolan-2-yl)benzene) (PDDTB),poly(tetrahydrobenzodithiophene) (PTHBDT),poly[1,2,4,5-tetrakis(propylthio)benzene] (PTKPTB, orpoly[3,4(ethylenedithio)thiophene] (PEDTT).

In other embodiments, the cathode active material layer contains anorganic material selected from a phthalocyanine compound, such as copperphthalocyanine, zinc phthalocyanine, tin phthalocyanine, ironphthalocyanine, lead phthalocyanine, nickel phthalocyanine, vanadylphthalocyanine, fluorochromium phthalocyanine, magnesium phthalocyanine,manganous phthalocyanine, dilithium phthalocyanine, aluminumphthalocyanine chloride, cadmium phthalocyanine, chlorogalliumphthalocyanine, cobalt phthalocyanine, silver phthalocyanine, ametal-free phthalocyanine, a chemical derivative thereof, or acombination thereof.

The cathode active material is preferably in a form of nanoparticle(spherical, ellipsoidal, and irregular shape), nanowire, nanofiber,nanotube, nanosheet, nanobelt, nanoribbon, nanodisc, nanoplatelet, ornanohorn having a thickness or diameter less than 100 nm. These shapescan be collectively referred to as “particles” unless otherwisespecified or unless a specific type among the above species is desired.Further preferably, the cathode active material has a dimension lessthan 50 nm, even more preferably less than 20 nm, and most preferablyless than 10 nm. In some embodiments, one particle or a cluster ofparticles may be coated with or embraced by a layer of carbon disposedbetween the particle(s) and/or a sulfonated elastomer composite layer(an encapsulating shell).

The cathode layer may further contain a graphite, graphene, or carbonmaterial mixed with the cathode active material particles. The carbon orgraphite material is selected from polymeric carbon, amorphous carbon,chemical vapor deposition carbon, coal tar pitch, petroleum pitch,mesophase pitch, carbon black, coke, acetylene black, activated carbon,fine expanded graphite particle with a dimension smaller than 100 nm,artificial graphite particle, natural graphite particle, or acombination thereof. Graphene may be selected from pristine graphene,graphene oxide, reduced graphene oxide, graphene fluoride, hydrogenatedgraphene, nitrogenated graphene, functionalized graphene, etc.

The cathode active material particles may be coated with or embraced bya conductive protective coating, selected from a carbon material,graphene, electronically conductive polymer, conductive metal oxide, orconductive metal coating.

The present invention also provides a lithium metal-air batterycomprising an air cathode, an anode comprising the anode-protectinglayer as defined above and disposed between the anode and the aircathode without using a conventional porous separator or membrane. Inthe air cathode, oxygen from the open air (or from an oxygen supplierexternal to the battery) is the primary cathode active material. The aircathode needs an inert material to support the lithium oxide materialformed at the cathode. The applicants have surprisingly found that anintegrated structure of conductive nanofilaments can be used as an aircathode intended for supporting the discharge product (e.g., lithiumoxide).

Hence, a further embodiment of the present invention is a lithiummetal-air battery, wherein the air cathode comprises an integratedstructure of electrically conductive nanometer-scaled filaments that areinterconnected to form a porous network of electron-conducting pathscomprising interconnected pores, wherein the filaments have a transversedimension less than 500 nm (preferably less than 100 nm). Thesenanofilaments can be selected from carbon nanotubes (CNTs), carbonnanofibers (CNFs), graphene sheets, carbon fibers, graphite fibers, etc.

The invention also provides a method of manufacturing a lithium battery,the method comprising: (a) providing a cathode active material layer andan optional cathode current collector to support the cathode activematerial layer; (b) providing an anode active material layer (containinga lithium metal or lithium alloy foil or coating) and an optional anodecurrent collector to support the lithium metal or lithium alloy foil orcoating; (c) providing an electrolyte in contact with the anode activematerial layer and the cathode active material layer without using aseparator to electrically separate the anode and the cathode; (d)providing an anode-protecting layer of an elastomer having a recoverabletensile elastic strain from 2% to 1,000% (preferably from 5% to 300%), alithium ion conductivity no less than 10⁻⁸ S/cm at room temperature, anda thickness from 1 nm to 100 μm (preferably from 10 nm to 10 μm). Thisanode-protecting layer is disposed between the lithium metal or lithiumalloy foil/coating and the cathode.

The invention also provides a method of improving the cycle-life of alithium metal secondary battery (not including a lithium-sulfur batteryor lithium-selenium battery). The method comprises implementing ananode-protecting layer between an anode active material layer and acathode electrode without using a porous separator. The anode-protectinglayer comprises an elastomer having a recoverable tensile elastic strainfrom 2% to 1,000% (preferably from 5% to 300%), a lithium ionconductivity no less than 10⁻⁸ S/cm (preferably >10⁻⁵ S/cm) at roomtemperature, and a thickness from 1 nm to 100 μm (preferably from 10 nmto 10 μm).

In some embodiments, the elastomer contains a material selected from asulfonated or non-sulfonated version of natural polyisoprene, syntheticpolyisoprene, polybutadiene, chloroprene rubber, polychloroprene, butylrubber, styrene-butadiene rubber, nitrile rubber, ethylene propylenerubber, ethylene propylene diene rubber, metallocene-basedpoly(ethylene-co-octene) elastomer, poly(ethylene-co-butene) elastomer,styrene-ethylene-butadiene-styrene elastomer, epichlorohydrin rubber,polyacrylic rubber, silicone rubber, fluorosilicone rubber,perfluoroelastomers, polyether block amides, chlorosulfonatedpolyethylene, ethylene-vinyl acetate, thermoplastic elastomer, proteinresilin, protein elastin, ethylene oxide-epichlorohydrin copolymer,polyurethane, urethane-urea copolymer, or a combination thereof.

In the above-defined method, the step implementing an anode-protectinglayer may be conducted by depositing a layer of an elastomer onto oneprimary surface of the anode active material layer. This step comprisesoptionally compressing the protected anode to improve a contact betweenthe anode-protecting layer and the anode active material layer, followedby combining the protected anode and the cathode together to form thelithium metal secondary battery. A good contact between the anode activematerial layer and the anode-protecting layer is essential to reducinginternal resistance.

In certain embodiments, the step of implementing the anode-protectinglayer is conducted by (i) preparing an anode active material layer; (ii)preparing a free-standing layer of an elastomer; and (iii) combining theanode active material layer, the elastomer layer, a cathode, and anon-solid state electrolyte together to form the lithium metal secondarybattery. A compressive stress may be advantageously applied (e.g. viapress-rolling) to improve the contact between the anode-protecting layerand the anode active material layer to be protected.

Preferably, the elastomer layer has a lithium-ion conductivity from 10⁻⁵S/cm to 5×10⁻² S/cm. In some embodiments, the elastomer has arecoverable tensile strain from 10% to 300% (more preferably >30%, andfurther more preferably >50%).

In certain embodiments, the procedure of providing an elastomer containsproviding a mixture/blend/composite of an elastomer (sulfonated orun-sulfonated) with a lithium-ion conducting material, a reinforcementmaterial (e.g. glass fibers, polymer fibers, etc.), or a combinationthereof.

In this mixture/blend/composite, the lithium ion-conducting material isdispersed in the elastomer and is preferably selected from Li₂CO₃, Li₂O,Li₂C₂O₄, LiOH, LiX, ROCO₂Li, HCOLi, ROLi, (ROCO₂Li)₂, (CH₂OCO₂Li)₂,Li₂S, Li_(x)SO_(y), or a combination thereof, wherein X═F, Cl, I, or Br,R=a hydrocarbon group, 0<x≤1 and 1≤y≤4.

In some embodiments, the lithium ion-conducting material is dispersed inthe sulfonated elastomer composite and is selected from lithiumperchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithiumborofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-methanesulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimidelithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃),Li-fluoroalkyl-phosphate (LiPF₃(CF₂CF₃)₃), lithiumbisperfluoro-ethylsulfonylimide (LiBETI), lithiumbis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide,lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-basedlithium salt, or a combination thereof.

The anode-protecting layer implemented between the anode active layerand the cathode is mainly for the purpose of reducing or eliminating thelithium metal dendrite by providing a more stable Li metal-electrolyteinterface that is more conducive to uniform deposition of Li metalduring battery charges. The anode-protecting layer also acts to blockthe penetration of any dendrite, if initiated, from reaching thecathode. The anode-protecting layer, being highly elastic, also canshrink or expands conformably, responsive to the thickness increase ordecrease of the anode active material layer. Other advantages willbecome more transparent later.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Schematic of a prior art lithium metal battery cell, containingan anode layer (a thin Li foil or Li coating deposited on a surface of acurrent collector, Cu foil), a porous separator, and a cathode activematerial layer, which is composed of particles of a cathode activematerial, a conductive additive (not shown) and a resin binder (notshown). A cathode current collector supporting the cathode active layeris also shown.

FIG. 2 Schematic of a presently invented lithium metal battery cellcontaining an anode layer (a thin Li foil or Li coating deposited on asurface of a current collector, Cu foil), a sulfonated elastomercomposite-based anode-protecting layer, a porous separator/electrolytelayer (or a layer of solid-state electrolyte), and a cathode activematerial layer, which is composed of particles of a cathode activematerial, a conductive additive (not shown) and a resin binder (notshown). A cathode current collector supporting the cathode active layeris also shown.

FIG. 3 The specific intercalation capacity curves of two lithium cells:one cell having a cathode containing V₂O₅ particles and a sulfonatedelastomer-based anode-protecting layer disposed between the anode activematerial layer (Li foil) and the cathode layer and the other cell havinga cathode containing graphene-embraced V₂O₅ particles, but having noanode-protecting protecting layer.

FIG. 4 The specific capacity values of two lithium-LiCoO₂ cells(initially the cell being lithium-free); one cell featuring ahigh-elasticity sulfonated elastomer layer at the anode and the othercell containing no anode protection layer.

FIG. 5 The discharge capacity curves of three coin cells having aFeF₃-based of cathode active materials: (1) one cell having ahigh-elasticity sulfonated elastomer-protected anode; (2) noanode-protecting layer; and (3) having double protection layers for theanode.

FIG. 6 Specific capacities of two lithium-FePc (organic) cells, eachhaving Li foil as an anode active material and FePc/RGO mixtureparticles as the cathode active material (one cell containing doublelayer-protected anode and the other no anode protection layer).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention is directed at a lithium metal secondary battery, whichis preferably based on an organic electrolyte, a polymer gelelectrolyte, an ionic liquid electrolyte, or a quasi-solid electrolyte;all these are non-solid-state electrolytes. The shape of a lithium metalsecondary battery can be cylindrical, square, button-like, etc. Thepresent invention is not limited to any battery shape or configurationor any type of electrolyte. The invented lithium secondary battery doesnot include a lithium-sulfur cell or lithium-selenium cell.

The invention provides a lithium metal secondary battery, comprising acathode, an anode, an anode-protecting layer disposed between thecathode and the anode, and a non-solid-state electrolyte.

In certain embodiments, the anode comprises: (a) a layer of lithium orlithium alloy (in the form of a foil, coating, or multiple particlesaggregated together) as an anode active material layer; and (b) ananode-protecting layer, in contact with the anode active material layer,having a thickness from 1 nm to 100 μm and comprising an elastomerhaving a fully recoverable tensile elastic strain from 2% to 1,000%, alithium ion conductivity from 10⁻⁸ S/cm to 5×10⁻² S/cm when measured atroom temperature.

The foil or coating of lithium or lithium alloy, as the anode activematerial layer or electrode, may be supported by a current collector(e.g. a Cu foil, a Ni foam, a porous layer of nanofilaments, such asmembrane, paper, or fabric of graphene sheets, carbon nanofibers, carbonnanotubes, etc. forming a 3D interconnected network ofelectron-conducting pathways).

Preferably, the anode-protecting layer (i.e. the elastomer layer) has alithium ion conductivity no less than 10⁻⁶ S/cm (typically and desirablyfrom 10⁻⁵ S/cm to 5×10⁻² S/cm, measured at room temperature), and athickness from 10 nm to 20 μm. These conditions are more amenable toallowing lithium ions to migrate in and out of the elastomer layerwithout much resistance.

Preferably, the elastomer contains a sulfonated or non-sulfonatedversion of an elastomer selected from natural polyisoprene, syntheticpolyisoprene, polybutadiene, chloroprene rubber, polychloroprene, butylrubber, styrene-butadiene rubber, nitrile rubber, ethylene propylenerubber, ethylene propylene diene rubber, metallocene-basedpoly(ethylene-co-octene) (POE) elastomer, polyethylene-co-butene) (PBE)elastomer, styrene-ethylene-butadiene-styrene (SEBS) elastomer,epichlorohydrin rubber, polyacrylic rubber, silicone rubber,fluorosilicone rubber, perfluoroelastomers, polyether block amides,chlorosulfonated polyethylene, ethylene-vinyl acetate, thermoplasticelastomer, protein resilin, protein elastin, ethyleneoxide-epichlorohydrin copolymer, polyurethane, urethane-urea copolymer,or a combination thereof.

Preferably, the anode-protecting layer (the elastomer layer) isdifferent in composition than the electrolyte per se used in the lithiumbattery and maintains as a discrete layer (not to be dissolved in theelectrolyte) that is disposed between the anode active material layer(e.g. Li foil) and the cathode. The anode-protecting layer may contain aliquid electrolyte that permeates or impregnates into the sulfonated ornon-sulfonated elastomer.

We have discovered that the anode-protecting layer provides severalunexpected benefits: (a) the formation of dendrite has been essentiallyeliminated; (b) uniform deposition of lithium back to the anode side isreadily achieved; (c) the layers ensure smooth and uninterruptedtransport of lithium ions from/to the lithium foil/coating and throughthe interface between the lithium foil/coating and the protective layerwith minimal interfacial resistance; (d) significant reduction in theamount of dead lithium particles near the Li foil; and (e) cyclestability can be significantly improved and cycle life increased.

In a conventional lithium metal cell, as illustrated in FIG. 1, theanode active material (lithium) is deposited in a thin film form or athin foil form directly onto an anode current collector (e.g. a Cufoil). The battery is a lithium metal battery, lithium sulfur battery,lithium-air battery, lithium-selenium battery, etc. As previouslydiscussed in the Background section, these lithium secondary batterieshave the dendrite-induced internal shorting and “dead lithium” issues atthe anode.

We have solved these challenging issues that have troubled batterydesigners and electrochemists alike for more than 30 years by developingand implementing two anode-protecting layers disposed between thelithium foil/coating and the separator layer. As schematically shown inFIG. 2, one embodiment of the present invention is a lithium metalbattery cell containing an anode layer (a thin Li foil or Li coatingdeposited on a surface of a current collector, such as a layer ofgraphene foam or a sheet of Cu foil), one anode-protecting layer, and acathode active material layer, which is composed of particles of acathode active material, a conductive additive (not shown) and a resinbinder (not shown). A cathode current collector (e.g. Al foil)supporting the cathode active layer is also shown in FIG. 2. The lithiummetal or alloy in the anode may be in a form of particles (e.g.surface-protected or surface-stabilized particles of Li or Li alloy).

The elastomer exhibits an elastic deformation of at least 2% whenmeasured under uniaxial tension. In the field of materials science andengineering, the “elastic deformation” is defined as a deformation of amaterial (when being mechanically stressed) that is essentially fullyrecoverable upon release of the load and the recovery is essentiallyinstantaneous. The elastic deformation is preferably greater than 5%,more preferably greater than 10%, further more preferably greater than30%, and still more preferably greater than 100% but less than 500%.

It may be noted that although FIG. 2 shows a lithium coating preexistingat the anode when the lithium battery is made, this is but one ofseveral embodiments of the instant invention. An alternative embodimentis a lithium battery that does not contain a lithium foil or lithiumcoating at the anode (only an anode current collector, such as a Cu foilor a graphene/CNT mat) when the battery is made. The needed amount oflithium to be bounced back and forth between the anode and the cathodeis initially stored in the cathode active material (e.g. lithiumvanadium oxide Li_(x)V₂O₅, instead of vanadium oxide, V₂O₅; or lithiumtransition metal oxide or phosphate, instead of, say, MoS₂). During thefirst charging procedure of the lithium battery (e.g. as part of theelectrochemical formation process), lithium comes out of the cathodeactive material, migrates to the anode side, and deposits on the anodecurrent collector. The presence of the presently invented protectivelayer enables uniform deposition of lithium ions on the anode currentcollector surface. Such an alternative battery configuration avoids theneed to have a layer of lithium foil or coating being present duringbattery fabrication. Bare lithium metal is highly sensitive to airmoisture and oxygen and, thus, is more challenging to handle in a realbattery manufacturing environment. This strategy of prestoring lithiumin the lithiated (lithium-containing) cathode active materials, such asLi_(x)V₂O₅ and Li₂S_(x), makes all the materials safe to handle in areal manufacturing environment. Cathode active materials, such asLi_(x)V₂O₅ and Li₂S_(x), are typically less air-sensitive.

The presently invented lithium secondary batteries can contain a widevariety of cathode active materials. The cathode active material layermay contain a cathode active material selected from an inorganicmaterial, an organic material, a polymeric material, or a combinationthereof. The inorganic material may be selected from a metal oxide,metal phosphate, metal silicide, metal selenide, transition metalsulfide, or a combination thereof.

The inorganic cathode active material may be selected from a lithiumcobalt oxide, lithium nickel oxide, lithium manganese oxide, lithiumvanadium oxide, lithium-mixed metal oxide, lithium iron phosphate,lithium manganese phosphate, lithium vanadium phosphate, lithium mixedmetal phosphate, lithium metal silicide, or a combination thereof.

In certain preferred embodiments, the inorganic material as a cathodeactive material for the lithium battery is selected from a metalfluoride or metal chloride including the group consisting of CoF₃, MnF₃,FeF₃, VF₃, VOF₃, TiF₃, BiF₃, NiF₂, FeF₂, CuF₂, CuF, SnF₂, AgF, CuCl₂,FeCl₃, MnCl₂, and combinations thereof. In certain preferredembodiments, the inorganic material is selected from a lithiumtransition metal silicate, denoted as Li₂MSiO₄ or Li₂Ma_(x)Mb_(y)SiO₄,wherein M and Ma are selected from Fe, Mn, Co, Ni, V, or VO; Mb isselected from Fe, Mn, Co, Ni, V, Ti, Al, B, Sn, or Bi; and x+y≤1.

In certain preferred embodiments, the inorganic material as a cathodeactive material is selected from a transition metal dichalcogenide, atransition metal trichalcogenide, or a combination thereof. Theinorganic material is selected from TiS₂, TaS₂, MoS₂, NbSe₃, MnO₂, CoO₂,an iron oxide, a vanadium oxide, or a combination thereof.

The cathode active material layer may contain a metal oxide containingvanadium oxide selected from the group consisting of VO₂, Li_(x)VO₂,V₂O₅, Li_(x)V₂O₅, V₃O₈, Li_(x)V₃O₈, Li_(x)V₃O₇, V₄O₉, Li_(x)V₄O₉, V₆O₁₃,Li_(x)V₆O₁₃, their doped versions, their derivatives, and combinationsthereof, wherein 0.1<x<5.

The cathode active material layer may contain a metal oxide or metalphosphate, selected from a layered compound LiMO₂, spinel compoundLiM₂O₄, olivine compound LiMPO₄, silicate compound Li₂MSiO₄, Tavoritecompound LiMPO₄F, borate compound LiMBO₃, or a combination thereof,wherein M is a transition metal or a mixture of multiple transitionmetals.

In some embodiments, the inorganic material is selected from: (a)bismuth selenide or bismuth telluride, (b) transition metaldichalcogenide or trichalcogenide, (c) sulfide, selenide, or tellurideof niobium, zirconium, molybdenum, hafnium, tantalum, tungsten,titanium, cobalt, manganese, iron, nickel, or a transition metal; (d)boron nitride, or (e) a combination thereof.

The cathode active material layer may contain an organic material orpolymeric material selected from Poly(anthraquinonyl sulfide) (PAQS), alithium oxocarbon, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA),poly(anthraquinonyl sulfide), pyrene-4,5,9,10-tetraone (PYT),polymer-bound PYT, Quino(triazene), redox-active organic material,Tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE),2,3,6,7,10,11-hexamethoxytriphenylene (HMTP), poly(5-amino-1,4-dyhydroxyanthraquinone) (PADAQ), phosphazene disulfide polymer ([(NPS₂)₃]n),lithiated 1,4,5,8-naphthalenetetraol formaldehyde polymer,Hexaazatrinaphtylene (HATN), Hexaazatriphenylene hexacarbonitrile(HAT(CN)₆), 5-Benzylidene hydantoin, Isatine lithium salt, Pyromelliticdiimide lithium salt, tetrahydroxy-p-benzoquinone derivatives (THQLi₄),N,N′-diphenyl-2,3,5,6-tetraketopiperazine (PHP),N,N′-diallyl-2,3,5,6-tetraketopiperazine (AP),N,N′-dipropyl-2,3,5,6-tetraketopiperazine (PRP), a thioether polymer, aquinone compound, 1,4-benzoquinone, 5,7,12,14-pentacenetetrone (PT),5-amino-2,3-dihydro-1,4-dyhydroxy anthraquinone (ADDAQ),5-amino-1,4-dyhydroxy anthraquinone (ADAQ), calixquinone, Li₄C₆O₆,Li₂C₆O₆, Li₆C₆O₆, or a combination thereof.

The thioether polymer is selected fromPoly[methanetetryl-tetra(thiomethylene)] (PMTTM),Poly(2,4-dithiopentanylene) (PDTP), a polymer containingPoly(ethene-1,1,2,2-tetrathiol) (PETT) as a main-chain thioetherpolymers, a side-chain thioether polymer having a main-chain consistingof conjugating aromatic moieties, and having a thioether side chain as apendant, Poly(2-phenyl-1,3-dithiolane) (PPDT), Poly(1,4-di(1,3-dithiolan-2-yl)benzene) (PDDTB), poly(tetrahydrobenzodithiophene)(PTHBDT), poly[1,2,4,5-tetrakis(propylthio)benzene] (PTKPTB, orpoly[3,4(ethylenedithio)thiophene] (PEDTT).

In other embodiments, the cathode active material layer contains anorganic material selected from a phthalocyanine compound, such as copperphthalocyanine, zinc phthalocyanine, tin phthalocyanine, ironphthalocyanine, lead phthalocyanine, nickel phthalocyanine, vanadylphthalocyanine, fluorochromium phthalocyanine, magnesium phthalocyanine,manganous phthalocyanine, dilithium phthalocyanine, aluminumphthalocyanine chloride, cadmium phthalocyanine, chlorogalliumphthalocyanine, cobalt phthalocyanine, silver phthalocyanine, ametal-free phthalocyanine, a chemical derivative thereof, or acombination thereof.

Preferably and typically, the elastomer has a lithium ion conductivityno less than 10⁻⁵ S/cm, more preferably no less than 10⁻⁴ S/cm, furtherpreferably no less than 10⁻³ S/cm, and most preferably no less than 10⁻²S/cm. In some embodiments, the elastomer comprises from 0.1% to 50%(preferably 1% to 35%) by weight of a lithium ion-conducting additivedispersed in an elastomer matrix material. The elastomer must have ahigh elasticity (elastic deformation strain value >2%). An elasticdeformation is a deformation that is fully recoverable and the recoveryprocess is essentially instantaneous (no significant time delay). Theelastomer composite can exhibit an elastic deformation from 2% up to1,000% (10 times of its original length), more typically from 5% to500%, and further more typically from 10% to 300%, and most typicallyand desirably from 30% to 300%. It may be noted that although a metaltypically has a high ductility (i.e. can be extended to a large extentwithout breakage), the majority of the deformation is plasticdeformation (non-recoverable) and only a small amount of elasticdeformation (typically <1% and more typically <0.2%).

Further, we have unexpectedly discovered that the presence of an amountof a lithium salt or sodium salt (1-35% by weight) and a liquid solvent(0-50%) can significantly increase the lithium-ion or sodium ionconductivity.

It is also advantageous to disperse a high-strength reinforcementmaterial in the anode-protecting material to increase the strength anddendrite-penetrating strength of the elastomer layer. Suitablereinforcement materials include glass fibers, ceramic fibers (e.g.silicon carbide fibers), polymer fibers (e.g. aromatic polyamide fiberssuch as Kevlar fibers, nylon fibers, ultrahigh molecular weightpolyethylene or UHMW-PE fibers, etc.), and ceramic discs, etc.

Typically, an elastomer is originally in a monomer or oligomer statesthat can be cured to form a cross-linked polymer that is highly elastic.Prior to curing, these polymers or oligomers are soluble in an organicsolvent to form a polymer solution. An ion-conducting additive or areinforcement may be added to this solution to form a suspension. Thissolution or suspension can then be formed into a thin layer of polymerprecursor on a surface of an anode current collector or a surface of aLi foil. The polymer precursor (monomer or oligomer and initiator) isthen polymerized and cured to form a lightly cross-linked polymer. Thisthin layer of polymer may be tentatively deposited on a solid substrate(e.g. surface of a polymer or glass), dried, and separated from thesubstrate to become a free-standing polymer layer. This free-standinglayer is then laid on a lithium foil/coating or implemented between alithium film/coating and a cathode layer. Polymer layer formation can beaccomplished by using one of several procedures well-known in the art;e.g. spraying, spray-painting, printing, coating, extrusion-basedfilm-forming, casting, etc.

One may dispense and deposit a layer of a sulfonated or un-sulfonatedelastomer onto a primary surface of the anode active material layer.Alternatively, one may dispense and deposit a layer of an elastomer ontoa primary surface of a cathode active material layer. Furtheralternatively, one may prepare a separate free-standing discrete layerof the elastomer first. This elastomer layer is then laminated betweenan anode active material layer and a cathode layer to form a batterycell.

Thus, the invention also provides a method of manufacturing a lithiumbattery, the method comprising: (a) providing a cathode active materiallayer and an optional cathode current collector to support the cathodeactive material layer; (b) providing an anode active material layer(containing a lithium metal or lithium alloy foil or coating) and anoptional anode current collector to support the lithium metal or lithiumalloy foil or coating; (c) providing an anode-protecting layer of anelastomer having a recoverable tensile elastic strain from 2% to 1,000%(preferably from 5% to 300%), a lithium ion conductivity no less than10⁻⁸ S/cm at room temperature, and a thickness from 1 nm to 100 μm(preferably from 10 nm to 10 μm), wherein the anode-protecting layer isdisposed between the cathode active material layer and the anode activematerial layer and in physical contact therewith (in physical contactwith both the anode active material and the cathode active materiallayer); and (d) providing an electrolyte in contact with the anodeactive material layer and the cathode active material layer (noadditional separator between the anode and the cathode).

The invention also provides a method of improving the cycle-life of alithium metal secondary battery (not including a lithium-sulfur batteryor lithium-selenium battery). The method comprises implementing anelastomer-based, lithium ion-conducting anode-protecting layer betweenan anode active material layer and a cathode active material layerwithout using a porous separator.

It may be noted that the presently invented lithium secondary batterycomprises at least the following layers: an optional anode currentcollector (e.g. a Cu foil or a graphene foam), an anode active materiallayer (e.g. a discrete lithium foil, a lithium coating layer, or a layerof lithium particles) supported by the anode current collector (ifpresent), an anode-protecting layer (elastomer or elastomer composite)substantially fully covering the anode active material layer, anelectrolyte but no porous separator or membrane, a cathode activematerial layer, and an optional cathode current collector (e.g. Al foil,graphene paper sheet, etc.). The electrolyte contains some liquidelectrolyte.

There are many different sequences with which these individual layersmay be produced and combined together. For instance, one may produce allcomponents in a free-standing form and then combine them together.Alternatively, one may produce certain components in singlefree-standing films but other components in a 2-layer or 3-layerstructure, followed by combining these components and structurestogether. For instance, one may spray, cast, or coat an elastomer layeronto a primary surface of a cathode layer to form a two-layer structure.This two-layer structure is then laminated with other components (e.g.an anode active material layer, standing alone or coated on a Cu foil)to form a battery cell.

Alternatively, the step of implementing an anode-protecting layer may beconducted by depositing a layer of an elastomer onto one primary surfaceof an anode active material layer. This step includes optionallycompressing the protected anode to improve the contact between theanode-protecting layer and the anode active material layer, followed bycombining the protected anode and the cathode together to form a lithiummetal secondary battery. A good contact between the anode activematerial layer and the anode-protecting layer is essential to reducinginternal resistance.

In certain embodiments, the step of implementing an anode-protectinglayer is conducted by forming a protecting layer of elastomer, followedby laminating the anode active material layer, the elastomer layer, thecathode layer, along with the electrolyte to form the lithium metalsecondary battery, wherein an optional (but desirable) compressivestress is applied to improve the contact between the anode-protectinglayer and the anode active material layer and/or the cathode activematerial layer during or after this laminating step.

Sulfonation of an elastomer or rubber may be accomplished by exposingthe elastomer/rubber to a sulfonation agent in a solution state or meltstate, in a batch manner or in a continuous process. The sulfonatingagent may be selected from sulfuric acid, sulfonic acid, sulfurtrioxide, chlorosulfonic acid, a bisulfate, a sulfate (e.g. zincsulfate, acetyl sulfate, etc.), a mixture thereof, or a mixture thereofwith another chemical species (e.g. acetic anhydride, thiolacetic acid,or other types of acids, etc.). In addition to zinc sulfate, there are awide variety of metal sulfates that may be used as a sulfonating agent;e.g. those sulfates containing Mg, Ca, Co, Li, Ba, Na, Pb, Ni, Fe, Mn,K, Hg, Cr, and other transition metals, etc.

For instance, a triblock copolymer, poly(styrene-isobutylene-styrene) orSIBS, may be sulfonated to several different levels ranging from 0.36 to2.04 mequiv./g (13 to 82 mol % of styrene; styrene being 19 mol % of theunsulfonated block copolymer). Sulfonation of SIBS may be performed insolution with acetyl sulfate as the sulfonating agent. First, aceticanhydride reacts with sulfuric acid to form acetyl sulfate (asulfonating agent) and acetic acid (a by-product). Then, excess water isremoved since anhydrous conditions are required for sulfonation of SIBS.The SIBS is then mixed with the mixture of acetyl sulfate and aceticacid. Such a sulfonation reaction produces sulfonic acid substituted tothe para-position of the aromatic ring in the styrene block of thepolymer. Elastomers having an aromatic ring may be sulfonated in asimilar manner.

A sulfonated elastomer also may be synthesized by copolymerization of alow level of functionalized (i.e. sulfonated) monomer with anunsaturated monomer (e.g. olefinic monomer, isoprene monomer oroligomer, butadiene monomer or oligomer, etc.).

A broad array of elastomers can be sulfonated to become sulfonatedelastomers. The elastomeric material may be selected from naturalpolyisoprene (e.g. cis-1,4-polyisoprene natural rubber (NR) andtrans-1,4-polyisoprene gutta-percha), synthetic polyisoprene (IR forisoprene rubber), polybutadiene (BR for butadiene rubber), chloroprenerubber (CR), polychloroprene (e.g. Neoprene, Baypren etc.), butyl rubber(copolymer of isobutylene and isoprene, IIR), including halogenatedbutyl rubbers (chloro butyl rubber (CIIR) and bromo butyl rubber (BIIR),styrene-butadiene rubber (copolymer of styrene and butadiene, SBR),nitrile rubber (copolymer of butadiene and acrylonitrile, NBR), EPM(ethylene propylene rubber, a copolymer of ethylene and propylene), EPDMrubber (ethylene propylene diene rubber, a terpolymer of ethylene,propylene and a diene-component), epichlorohydrin rubber (ECO),polyacrylic rubber (ACM, ABR), silicone rubber (SI, Q, VMQ),fluorosilicone rubber (FVMQ), fluoroelastomers (FKM, and FEPM; such asViton, Tecnoflon, Fluorel, Aflas and Dai-El), perfluoroelastomers (FFKM:Tecnoflon PFR, Kalrez, Chemraz, Perlast), polyether block amides (PEBA),chlorosulfonated polyethylene (CSM; e.g. Hypalon), and ethylene-vinylacetate (EVA), thermoplastic elastomers (TPE), protein resilin, proteinelastin, ethylene oxide-epichlorohydrin copolymer, polyurethane,urethane-urea copolymer, and combinations thereof.

In some embodiments, an elastomer can form a polymer matrix compositecontaining a lithium ion-conducting additive dispersed in the elastomermatrix material, wherein the lithium ion-conducting additive is selectedfrom Li₂CO₃, Li₂O, Li₂C₂O₄, LiOH, LiX, ROCO₂Li, HCOLi, ROLi, (ROCO₂Li)₂,(CH₂OCO₂Li)₂, Li₂S, Li_(x)SO_(y), or a combination thereof, wherein X═F,Cl, I, or Br, R=a hydrocarbon group, 0<x≤1 and 1≤y≤4.

In some embodiments, the elastomer can be mixed with a lithiumion-conducting additive, which contains a lithium salt selected fromlithium perchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆),lithium borofluoride (LiBF₄), lithium hexafluoroarsenide (LiAsF₆),lithium trifluoro-methanesulfonate (LiCF₃SO₃), bis-trifluoromethylsulfonylimide lithium (LiN(CF₃SO₂)₂), lithium bis(oxalato)borate(LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate(LiNO₃), Li-fluoroalkyl-phosphate (LiPF₃(CF₂CF₃)₃), lithiumbisperfluoro-ethylsulfonylimide (LiBETI), lithiumbis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide,lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-basedlithium salt, or a combination thereof.

In some embodiments, the elastomer may form a mixture, co-polymer, orsemi-interpenetrating network with a lithium ion-conducting polymerselected from poly(ethylene oxide) (PEO), polypropylene oxide (PPO),poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA),poly(vinylidene fluoride) (PVDF), poly bis-methoxyethoxyethoxide-phosphazene, polyvinyl chloride, polydimethylsiloxane,poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a derivativethereof (e.g. sulfonated versions), or a combination thereof.

The electrolyte for a lithium secondary cell may be an organicelectrolyte, ionic liquid electrolyte, gel polymer electrolyte,quasi-solid electrolyte (e.g. containing 2M-14 M of a lithium salt in asolvent) or a combination thereof. The electrolyte typically contains analkali metal salt (lithium salt, sodium salt, and/or potassium salt)dissolved in a solvent.

The solvent may be selected from 1,3-dioxolane (DOL),1,2-dimethoxyethane (DME), tetraethylene glycol dimethylether (TEGDME),poly(ethylene glycol) dimethyl ether (PEGDME), diethylene glycol dibutylether (DEGDBE), 2-ethoxyethyl ether (EEE), sulfone, sulfolane, ethylenecarbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC),diethyl carbonate (DEC), ethyl propionate, methyl propionate, propylenecarbonate (PC), gamma-butyrolactone (γ-BL), acetonitrile (AN), ethylacetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene,methyl acetate (MA), fluoroethylene carbonate (FEC), vinylene carbonate(VC), allyl ethyl carbonate (AEC), a hydrofluoroether, a roomtemperature ionic liquid solvent, or a combination thereof.

The electrolytic salts to be incorporated into a non-aqueous electrolytemay be selected from a lithium salt such as lithium perchlorate(LiClO₄), lithium hexafluorophosphate (LiPF₆), lithium borofluoride(LiBF₄), lithium hexafluoroarsenide (LiAsF₆), lithiumtrifluoro-methanesulfonate (LiCF₃SO₃), bis-trifluoromethyl sulfonylimidelithium [LiN(CF₃SO₂)₂], lithium bis(oxalato)borate (LiBOB), lithiumoxalyldifluoroborate (LiBF₂C₂O₄), lithium nitrate (LiNO₃),Li-fluoroalkyl-phosphates (LiPF3(CF₂CF₃)₃), lithiumbisperfluoroethysulfonylimide (LiBETI), an ionic liquid salt, sodiumperchlorate (NaClO₄), potassium perchlorate (KClO₄), sodiumhexafluorophosphate (NaPF₆), potassium hexafluorophosphate (KPF₆),sodium borofluoride (NaBF₄), potassium borofluoride (KBF₄), sodiumhexafluoroarsenide, potassium hexafluoroarsenide, sodiumtrifluoro-methanesulfonate (NaCF₃SO₃), potassiumtrifluoro-methanesulfonate (KCF₃SO₃), bis-trifluoromethyl sulfonylimidesodium (NaN(CF₃SO₂)₂), sodium trifluoromethanesulfonimide (NaTFSI), andbis-trifluoromethyl sulfonylimide potassium (KN(CF₃SO₂)₂). Among them,LiPF₆, LiBF₄ and LiN(CF₃SO₂)₂ are preferred for Li—S cells, NaPF₆ andLiBF₄ for Na—S cells, and KBF₄ for K-S cells. The content ofaforementioned electrolytic salts in the non-aqueous solvent ispreferably 0.5 to 3.0 M (mol/L) at the cathode side and 3.0 to >10 M atthe anode side.

The ionic liquid is composed of ions only. Ionic liquids are low meltingtemperature salts that are in a molten or liquid state when above adesired temperature. For instance, a salt is considered as an ionicliquid if its melting point is below 100° C. If the melting temperatureis equal to or lower than room temperature (25° C.), the salt isreferred to as a room temperature ionic liquid (RTIL). The IL salts arecharacterized by weak interactions, due to the combination of a largecation and a charge-delocalized anion. This results in a low tendency tocrystallize due to flexibility (anion) and asymmetry (cation).

A typical and well-known ionic liquid is formed by the combination of a1-ethyl-3-methylimidazolium (EMI) cation and anN,N-bis(trifluoromethane)sulfonamide (TFSI) anion. This combinationgives a fluid with an ionic conductivity comparable to many organicelectrolyte solutions and a low decomposition propensity and low vaporpressure up to ˜300-400° C. This implies a generally low volatility andnon-flammability and, hence, a much safer electrolyte for batteries.

Ionic liquids are basically composed of organic ions that come in anessentially unlimited number of structural variations owing to thepreparation ease of a large variety of their components. Thus, variouskinds of salts can be used to design the ionic liquid that has thedesired properties for a given application. These include, among others,imidazolium, pyrrolidinium and quaternary ammonium salts as cations andbis(trifluoromethanesulfonyl) imide, bis(fluorosulfonyl)imide, andhexafluorophosphate as anions. Based on their compositions, ionicliquids come in different classes that basically include aprotic, proticand zwitterionic types, each one suitable for a specific application.

Common cations of room temperature ionic liquids (RTILs) include, butnot limited to, tetraalkylammonium, di-, tri-, andtetra-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium,dialkylpiperidinium, tetraalkylphosphonium, and trialkylsulfonium.Common anions of RTILs include, but not limited to, BF₄ ⁻, B(CN)₄ ⁻,CH₃BF₃ ⁻, CH2CHBF₃ ⁻, CF₃BF₃ ⁻, C₂F₅BF₃ ⁻, n-C₃F₇BF₃ ⁻, n-C₄F₉BF₃ ⁻, PF₆⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻, N(SO₂F)₂ ⁻,N(CN)₂ ⁻, C(CN)₃ ⁻, SeCN⁻, CuCl₂ ⁻, AlCl₄ ⁻, F(HF)_(2.3) ⁻, etc.Relatively speaking, the combination of imidazolium- or sulfonium-basedcations and complex halide anions such as AlCl₄ ⁻, BF₄ ⁻, CF₃CO₂ ⁻,CF₃SO₃ ⁻, NTf₂ ⁻, N(SO₂F)₂ ⁻, or F(HF)_(2.3) ⁻ results in RTILs withgood working conductivities.

RTILs can possess archetypical properties such as high intrinsic ionicconductivity, high thermal stability, low volatility, low (practicallyzero) vapor pressure, non-flammability, the ability to remain liquid ata wide range of temperatures above and below room temperature, highpolarity, high viscosity, and wide electrochemical windows. Theseproperties, except for the high viscosity, are desirable attributes whenit comes to using an RTIL as an electrolyte ingredient (a salt and/or asolvent) in a lithium metal cell.

Example 1: Sulfonation of Triblock CopolymerPoly(Styrene-Isobutylene-Styrene) or SIBS

Both non-sulfonated and sulfonated elastomers are used to build theanode-protecting layer in the present invention. The sulfonated versionstypically provide a much higher lithium ion conductivity and, hence,enable higher-rate capability or higher power density. The elastomermatrix can contain a lithium ion-conducting additive, an electronicallynon-conducting reinforcement, and/or a lithium metal-stabilizingadditive.

An example of the sulfonation procedure used in this study for making asulfonated elastomer is summarized as follows: a 10% (w/v) solution ofSIBS (50 g) and a desired amount of graphene oxide sheets (0 to 40.5% bywt.) in methylene chloride (500 ml) was prepared. The solution wasstirred and refluxed at approximately 40° C., while a specified amountof acetyl sulfate in methylene chloride was slowly added to begin thesulfonation reaction. Acetyl sulfate in methylene chloride was preparedprior to this reaction by cooling 150 ml of methylene chloride in an icebath for approximately 10 min. A specified amount of acetic anhydrideand sulfuric acid was then added to the chilled methylene chloride understirring conditions. Sulfuric acid was added approximately 10 min afterthe addition of acetic anhydride with acetic anhydride in excess of a1:1 mole ratio. This solution was then allowed to return to roomtemperature before addition to the reaction vessel.

After approximately 5 h, the reaction was terminated by slowly adding100 ml of methanol. The reacted polymer solution was then precipitatedwith deionized water. The precipitate was washed several times withwater and methanol, separately, and then dried in a vacuum oven at 50°C. for 24 h. This washing and drying procedure was repeated until the pHof the wash water was neutral. After this process, the final polymeryield was approximately 98% on average. This sulfonation procedure wasrepeated with different amounts of acetyl sulfate to produce severalsulfonated polymers with various levels of sulfonation or ion-exchangecapacities (IECs). The mol % sulfonation is defined as: mol %=(moles ofsulfonic acid/moles of styrene)×100%, and the IEC is defined as themille-equivalents of sulfonic acid per gram of polymer (mequiv./g).

After sulfonation and washing of each polymer, the S-SIBS samples weredissolved in a mixed solvent of toluene/hexanol (85/15, w/w) withconcentrations ranging from 0.5 to 2.5% (w/v). Desired amounts ofKevlar® fibers (du Pont) and a lithium metal-stabilizing additives (e.g.LiNO₃ and lithium trifluoromethanesulfonimide) were then added into thesolution to form slurry samples. The slurry samples were slot-die coatedon a PET plastic substrate to form layers of sulfonated elastomercomposite. The lithium metal-stabilizing additives were found to impartstability to lithium metal-electrolyte interfaces.

Example 2: Synthesis of Sulfonated Polybutadiene (PB) by Free RadicalAddition of Thiolacetic Acid (TAA) Followed by In Situ Oxidation withPerformic Acid

A representative procedure is given as follows. PB (8.0 g) was dissolvedin toluene (800 mL) under vigorous stirring for 72 h at room temperaturein a 1 L round-bottom flask. Benzophenone (BZP) (0.225 g; 1.23 mmol;BZP/olefin molar ratio=1:120) and TAA (11.9 mL; 0.163 mol, TAA/olefinmolar ratio=1.1) and a desired amount of Nylon fibers (0%-40% by wt.)were introduced into the reactor, and the polymer solution wasirradiated for 1 h at room temperature with UV light of 365 nm and powerof 100 W.

The resulting thioacetylated polybutadiene (PB-TA)/Nylon fiber compositewas isolated by pouring 200 mL of the toluene solution in a plenty ofmethanol and the polymer recovered by filtration, washed with freshmethanol, and dried in vacuum at room temperature (Yield=3.54 g). Formicacid (117 mL; 3.06 mol; HCOOH/olefin molar ratio=25), along with adesired amount of anode active material particles, from 10 to 100 grams)were added to the toluene solution of PB-TA at 50° C. followed by slowaddition of 52.6 mL of hydrogen peroxide (35 wt %; 0.61 mol; H₂O₂/olefinmolar ratio=5) in 20 min. We would like to caution that the reaction isautocatalytic and strongly exothermic. The resulting slurry was cast toobtain sulfonated polybutadiene (PB-SA) composite layers. It may benoted that Nylon fibers or other additives may be added at differentstages of the procedure: before, during or after BZP is added.

Example 3: Synthesis of Sulfonated SBS

Sulfonated styrene-butadiene-styrene triblock copolymer (SBS) basedelastomer was directly synthesized. First, SBS (optionally along with alithium ion-conducting additive or electron-conducting additive) isfirst epoxidized by performic acid formed in situ, followed byring-opening reaction with an aqueous solution of NaHSO₃. In a typicalprocedure, epoxidation of SBS was carried out via reaction of SBS incyclohexane solution (SBS concentration=11 g/100 mL) with performic acidformed in situ from HCOOH and 30% aqueous H₂O₂ solution at 70° C. for 4h, using 1 wt. % poly(ethylene glycol)/SBS as a phase transfer catalyst.The molar ratio of H₂O₂/HCOOH was 1. The product (ESBS) was precipitatedand washed several times with ethanol, followed by drying in a vacuumdryer at 60° C.

Subsequently, ESBS was first dissolved in toluene to form a solutionwith a concentration of 10 g/100 mL, into which was added 5 wt. %TEAB/ESBS as a phase transfer catalyst and 5 wt. % DMA/ESBS as aring-opening catalyst. Herein, TEAB=tetraethyl ammonium bromide andDMA=N,N-dimethyl aniline. An aqueous solution of NaHSO₃ and Na₂SO₃(optionally along with an additive or reinforcement material, if notadded earlier) was then added with vigorous stirring at 60° C. for 7 hat a molar ratio of NaHSO₃/epoxy group at 1.8 and a weight ratio ofNa₂SO₃/NaHSO₃ at 36%. This reaction allows for opening of the epoxidering and attaching of the sulfonate group according to the followingreaction:

The reaction was terminated by adding a small amount of acetone solutioncontaining antioxidant. The mixture was washed with distilled water andthen precipitated by ethanol while being cast into thin films, followedby drying in a vacuum dryer at 50° C. It may be noted electronicallynon-conducting reinforcement (e.g. polymer fibers) and/or lithiumion-conducting additive (e.g. Li₂CO₃ and NaBF₄) may be added duringvarious stages of the aforementioned procedure (e.g. right from thebeginning, or prior to the ring opening reaction).

Example 4: Synthesis of Sulfonated SBS by Free Radical Addition ofThiolacetic Acid (TAA) Followed by In Situ Oxidation with Per-FormicAcid

A representative procedure is given as follows. SBS (8.000 g) in toluene(800 mL) was left under vigorous stirring for 72 hours at roomtemperature and heated later on for 1 h at 65° C. in a 1 L round-bottomflask until the complete dissolution of the polymer. Thus, benzophenone(BZP, 0.173 g; 0.950 mmol; BZP/olefin molar ratio=1:132) and TAA (8.02mL; 0.114 mol, TAA/olefin molar ratio=1.1) were added, and the polymersolution was irradiated for 4 h at room temperature with UV light of 365nm and power of 100 W. To isolate a fraction of the thioacetylatedsample (S(B-TA)S), 20 mL of the polymer solution was treated with plentyof methanol, and the polymer was recovered by filtration, washed withfresh methanol, and dried in vacuum at room temperature. The toluenesolution containing the thioacetylated polymer was equilibrated at 50°C., and 107.4 mL of formic acid (2.84 mol; HCOOH/olefin molarratio=27.5) and 48.9 mL of hydrogen peroxide (35 wt %; 0.57 mol;H₂O₂/olefin molar ratio=5.5) were added in about 15 min. It may becautioned that the reaction is autocatalytic and strongly exothermic!The non-conductive reinforcement material was added before or after thisreaction. The resulting slurry was stirred for 1 h, and then most of thesolvent was distilled off in vacuum at 35° C. Finally, the slurrycontaining the sulfonated elastomer, along with desired additives, wasadded with acetonitrile, cast into films, washed with freshacetonitrile, and dried in vacuum at 35° C. to obtain layers ofsulfonated elastomers.

Other elastomers (e.g. polyisoprene, EPDM, EPR, polyurethane, etc.) weresulfonated in a similar manner. Alternatively, all the rubbers orelastomers can be directly immersed in a solution of sulfuric acid, amixture of sulfuric acid and acetyl sulfate, or other sulfonating agentdiscussed above to produce sulfonated elastomers/rubbers. Again, desiredadditives or reinforcement materials may be added at various stages ofthe procedure.

Example 5: Lithium Battery Containing a Sulfonated Elastomer-ProtectedLithium Anode and a Cathode Containing V₂O₅ Particles

Cathode active material layers were prepared from V₂O₅ particles andgraphene-embraced V₂O₅ particles, respectively. The V₂O₅ particles werecommercially available. Graphene-embraced V₂O₅ particles were preparedin-house. In a typical experiment, vanadium pentoxide gels were obtainedby mixing V₂O₅ in a LiCl aqueous solution. The Lit exchanged gelsobtained by interaction with LiCl solution (the Li:V molar ratio waskept as 1:1) was mixed with a GO suspension and then placed in aTeflon-lined stainless steel 35 ml autoclave, sealed, and heated up to180° C. for 12 h. After such a hydrothermal treatment, the green solidswere collected, thoroughly washed, ultrasonicated for 2 minutes, anddried at 70° C. for 12 h followed by mixing with another 0.1% GO inwater, ultrasonicating to break down nanobelt sizes, and thenspray-drying at 200° C. to obtain graphene-embraced V₂O₅ compositeparticulates. Selected amounts of V₂O₅ particles and graphene-embracedV₂O₅ particles, respectively, were then each made into a cathode layerfollowing a well-known slurry coating process.

The sulfonated elastomer films for use as the anode-protecting layerwere SIBS as prepared in Example 1. Several tensile testing specimenswere cut from the film and tested with a universal testing machine. Theresults indicate that this series of sulfonated elastomer films have anelastic deformation from approximately 150% to 465%. The addition of upto 30% by weight of a reinforcement material (e.g. Kevlar fibers) and/oran inorganic additive typically reduces this elasticity down to areversible tensile strain from 6% to 110%.

For electrochemical testing, the working electrodes (cathode layers)were prepared by mixing 85 wt. % V₂O₅ or 88% of graphene-embraced V₂O₅particles, 5-8 wt. % CNTs, and 7 wt. % polyvinylidene fluoride (PVDF)binder dissolved in N-methyl-2-pyrrolidinoe (NMP) to form a slurry of 5wt. % total solid content. After coating the slurries on Al foil, theelectrodes were dried at 120° C. in vacuum for 2 h to remove the solventbefore pressing. Then, the electrodes were cut into a disk (ϕ=12 mm) anddried at 100° C. for 24 h in vacuum.

Electrochemical measurements were carried out using CR2032 (3V)coin-type cells with lithium metal as the counter electrode (actually ananode of a Li-transition metal oxide cell), Celgard 2400 membrane asseparator (for the cell containing no anode-protecting elastomer layer),and 1 M LiPF₆ electrolyte solution dissolved in a mixture of ethylenecarbonate (EC) and diethyl carbonate (DEC) (EC-DEC, 1:1 v/v). The cellassembly was performed in an argon-filled glove-box. The CV measurementswere carried out using a CH-6 electrochemical workstation at a scanningrate of 1 mV/s. The electrochemical performance of the cells wereevaluated by galvanostatic charge/discharge cycling at a current densityof 50 mA/g using an Arbin Electrochemical Testing Station.

Summarized in FIG. 3 are the specific intercalation capacity curves oftwo lithium cells: one cell having a cathode containing V₂O₅ particlesand a sulfonated elastomer-based anode-protecting layer disposed betweenthe anode active material layer (Li foil) and the cathode layer and theother cell having a cathode containing graphene-embraced V₂O₅ particles,but having no anode-protecting protecting layer. As the number of cyclesincreases, the specific capacity of the un-protected cells drops at amuch faster rate. In contrast, the presently invented approach of anelastomer-based anode-protecting layer (even without the use of aconventional porous separator) provides the battery cell with a stablecycling behavior. These data have clearly demonstrated the surprisingand superior performance of the presently invented anode protectionapproach for the lithium metal layer.

The sulfonated elastomer-based protective layer appears to be capable ofreversibly deforming to a great extent without breakage when the lithiumfoil decreases in thickness during battery discharge. The protectivelayer also prevent the continued reaction between liquid electrolyte andlithium metal at the anode, reducing the problem of continuing loss inlithium and electrolyte. This also enables a significantly more uniformdeposition of lithium ions upon returning from the cathode during abattery re-charge step; hence, no lithium dendrite. These were observedby using SEM to examine the surfaces of the electrodes recovered fromthe battery cells after some numbers of charge-discharge cycles.

Example 6: Sulfonated Elastomer Implemented in the Anode of aLithium-LiCoO₂ Cell (Initially the Cell Anode has an Ultra-Thin LithiumLayer, <1 μm Thick)

The sulfonated elastomer as a lithium-protecting layer was based on thesulfonated polybutadiene (PB) prepared according to a procedure used inExample 2. Tensile testing was also conducted on the sulfonatedelastomer films (without the conductive reinforcement material). Thisseries of sulfonated elastomers can be elastically stretched up toapproximately 135% (having some lithium salt or conductive reinforcementmaterial dispersed therein) or up to 770% (with no additive).

FIG. 4 shows the specific lithium intercalation capacity of twolithium-LiCoO₂ cells (initially the cell being lithium-free); one cellfeaturing a high-elasticity sulfonated elastomer layer at the anode andthe other cell containing no anode protection layer. These data indicatethat the cell having a sulfonated PB-based anode-protecting layer offerssignificantly more stable cycling behavior. The sulfonated elastomeralso acts to isolate the liquid electrolyte from the lithium coating yetstill allowing for easy diffusion of lithium ions.

Example 7: Li Metal Cells Containing Transition Metal FluorideNanoparticle-Based Cathode and a Sulfonated Elastomer-BasedAnode-Protecting Layer

This sulfonated elastomer layer was based on sulfonatedstyrene-butadiene-styrene triblock copolymer (SBS). Tensile testing wasconducted on some cut pieces of these layers. This series ofcross-linked polymers can be elastically stretched up to approximately820% (without any additive). The addition of additives results in anelasticity of approximately 5% (e.g. with 20% carbon black) to 160%(e.g. with 5% graphene sheets, as a conductive additive).

Commercially available powders of CoF₃, MnF₃, FeF₃, VF₃, VOF₃, TiF₃, andBiF₃ were subjected to high-intensity ball-milling to reduce theparticle size down to approximately 0.5-2.3 μm. Each type of these metalfluoride particles, along with graphene sheets (as a conductiveadditive), was then added into an NMP and PVDF binder suspension to forma multiple-component slurry. The slurry was then slurry-coated on Alfoil to form cathode layers.

Shown in FIG. 5 are the discharge capacity curves of two coin cellshaving the same cathode active material (FeF₃), but one cell having asulfonated elastomer-based anode-protecting layer, second cell having noprotective layer. These results have clearly demonstrated that theelastomer layer protection strategy provides the best protection againstcapacity decay of a lithium metal battery.

The elastomer layer appears to be capable of reversibly deformingwithout breakage when the anode layer expands and shrinks during chargeand discharge. The elastomer layer also prevents continued reactionbetween the liquid electrolyte and the lithium metal. No dendrite-likefeatures were found with the anode being protected by a sulfonatedelastomer composite. This was confirmed by using SEM to examine thesurfaces of the electrodes recovered from the battery cells after somenumbers of charge-discharge cycles.

Example 8: Li-Organic Cell Containing a Naphthalocyanine/ReducedGraphene Oxide (FePc/RGO) Particulate Cathode and a Protected Li FoilAnode

Particles of combined FePc/graphene sheets were obtained by ball-millinga mixture of FePc and RGO in a milling chamber for 30 minutes. Theresulting FePc/RGO mixture particles were potato-like in shape. Twolithium cells were prepared, each containing a Li foil anode, and acathode layer of FePc/RGO particles; one cell containing ananode-protecting layer without a porous separator, and the other havinga conventional porous separator layer but no anode-protecting layer.

The cycling behaviors of these 2 lithium cells are shown in FIG. 6,which indicates that the lithium-organic cell having a sulfonatedelastomer-based protection layer exhibits a significantly more stablecycling response. These protective layers reduce or eliminate theundesirable reactions between the lithium metal and the electrolyte, yetthe elastomer layer itself remains in ionic contact with the protectedlithium metal and is permeable to lithium ions. This approach hassignificantly increased the cycle life of all lithium-organic batteries.

Example 9: Effect of Lithium Ion-Conducting Additive in a SulfonatedElastomer Composite

A wide variety of lithium ion-conducting additives were added to severaldifferent polymer matrix materials to prepare anode protection layers.The lithium ion conductivity vales of the resulting complex materialsare summarized in Table 1 below. We have discovered that these compositematerials are suitable anode-protecting layer materials provided thattheir lithium ion conductivity at room temperature is no less than 10⁻⁶S/cm. With these materials, lithium ions appear to be capable of readilydiffusing through the protective layer having a thickness no greaterthan 1 μm. For thicker polymer films (e.g. 10 μm), a lithium ionconductivity at room temperature of these sulfonated elastomercomposites no less than 10⁻⁴ S/cm would be required.

TABLE 1 Lithium ion conductivity of various sulfonated elastomercomposite compositions as a lithium metal-protecting layer. Sample %sulfonated elastomer No. Lithium-conducting additive (1-2 μm thick)Li-ion conductivity (S/cm) E-1p Li₂CO₃ + (CH₂OCO₂Li)₂ 70-99% 1.3 × 10⁻⁴to 3.3 × 10⁻³ S/cm B1p LiF + LiOH + Li₂C₂O₄ 60-90% 4.2 × 10⁻⁵ to 2.6 ×10⁻³ S/cm B2p LiF + HCOLi 80-99% 1.2 × 10⁻⁴ to 1.4 × 10⁻³ S/cm B3p LiOH70-99% 8.5 × 10⁻⁴ to 1.1 × 10⁻² S/cm B4p Li₂CO₃ 70-99% 4.3 × 10⁻³ to 9.5× 10⁻³ S/cm B5p Li₂C₂O₄ 70-99% 8.2 × 10⁻⁴ to 1.3 × 10⁻² S/cm B6pLi₂CO₃ + LiOH 70-99% 1.5 × 10⁻³ to 1.7 × 10⁻² S/cm C1p LiClO₄ 70-99% 4.0× 10⁻⁴ to 2.2 × 10⁻³ S/cm C2p LiPF₆ 70-99% 2.1 × 10⁻⁴ to 6.2 × 10⁻³ S/cmC3p LiBF₄ 70-99% 1.2 × 10⁻⁴ to 1.7 × 10⁻³ S/cm C4p LiBOB + LiNO₃ 70-99%1.4 × 10⁻⁴ to 3.2 × 10⁻³ S/cm S1p Sulfonated polyaniline 85-99% 3.2 ×10⁻⁵ to 9.5 × 10⁻⁴ S/cm S2p Sulfonated PEEK 85-99% 1.4 × 10⁻⁴ to 1.3 ×10⁻³ S/cm S3p Sulfonated PVDF 80-99% 1.7 × 10⁻⁴ to 1.5 × 10⁻⁴ S/cm 54pPolyethylene oxide 80-99% 4.2 × 10⁻⁴ to 3.4 × 103⁴ S/cm

Example 10: Cycle Stability of Various Rechargeable Lithium BatteryCells

In lithium-ion battery industry, it is a common practice to define thecycle life of a battery as the number of charge-discharge cycles thatthe battery suffers a 20% decay in capacity based on the initialcapacity measured after the required electrochemical formation.Summarized in Table 2 below are the cycle life data of a broad array ofbatteries featuring an anode with or without an anode-protecting polymerlayer.

TABLE 2 Cycle life data of various lithium secondary (rechargeable)batteries. Initial Cycle life Anode-protecting Type & % of cathodeactive capacity (No. of Sample ID elastomer material (mAh/g) cycles)CuCl₂-1e sulfonated elastomer 85% by wt. CuCl₂ particles (80 537 1550nm) + 7% graphite + 8% binder CuCl₂-2e none 85% by wt. CuCl₂ particles(80 536 112 nm) + 7% graphite + 8% binder BiF₃-1e none 85% by wt. BiFe₃particles + 7% 275 115 graphene + 8% binder BiF₃-2e Sulfonatedelastomer + 85% by wt. BiFe₃ particles + 7% 276 1,405 50% ethylene oxidegraphene + 8% binder Li₂MnSiO₄-1e sulfonated elastomer 85% C-coatedLi₂MnSiO₄ + 7% 254 2,230 composite CNT + 8% binder Li₂MnSiO₄-2e none 85%C-coated Li₂MnSiO₄ + 7% 252 543 CNT + 8% binder Li₆C₆O₆-1e sulfonatedelastomer Li₆C₆O₆-graphene ball-milled 439 1,444 composite + 5% Kevlarfibers Li₆C₆O₆-2e none Li₆C₆O₆-graphene ball-milled 438 116 MoS₂-1esulfonated elastomer 85% MoS₂ + 8% graphite + 224 1,545 composite binderMoS₂-2e none 85% MoS₂ + 8% graphite + 225 156 binder

In conclusion, the anode protecting layer is surprisingly effective inalleviating the problems of lithium metal dendrite formation and lithiummetal-electrolyte reactions that otherwise lead to rapid capacity decayand potentially internal shorting and explosion of the lithium secondarybatteries. The elastomer layer appears to be capable of expanding orshrinking congruently or conformably with the anode active materiallayer. This capability helps to maintain a good contact between thecurrent collector (or the lithium film itself) and the protective layer,enabling uniform re-deposition of lithium ions without interruption.

We claim:
 1. A lithium metal secondary battery comprising a cathode, ananode, and a non-solid state electrolyte without a porous separatordisposed between said cathode and said anode, wherein said anodecomprises: a) an anode active material layer containing a layer oflithium or lithium alloy, in a form of a foil, coating, or multipleparticles aggregated together, as an anode active material; and b) ananode-protecting layer in physical contact with said anode activematerial layer, having a thickness from 1 nm to 100 μm and comprising anelastomer having a fully recoverable tensile elastic strain from 2% to1,000% and a lithium ion conductivity from 10⁻⁸ S/cm to 5×10⁻² S/cm whenmeasure at room temperature; wherein said lithium metal secondarybattery does not include a lithium-sulfur battery or lithium-seleniumbattery.
 2. The lithium metal secondary battery of claim 1, wherein saidnon-solid state electrolyte is selected from the group consisting oforganic liquid electrolyte, ionic liquid electrolyte, polymer gelelectrolyte, quasi-solid electrolyte having a lithium salt dissolved inan organic or ionic liquid with a lithium salt concentration higher than2.0 M, and combinations thereof.
 3. The lithium metal secondary batteryof claim 1, wherein said anode active material layer, saidanode-protecting layer, and said cathode are laminated together in suchmanner that the battery is under a compressive stress or strain.
 4. Thelithium metal secondary battery of claim 1, wherein said elastomercontains a material selected from non-sulfonated and sulfonated versionsselected from the group consisting of natural polyisoprene, syntheticpolyisoprene, polybutadiene, chloroprene rubber, polychloroprene, butylrubber, styrene-butadiene rubber, nitrile rubber, ethylene propylenerubber, ethylene propylene diene rubber, metallocene-basedpoly(ethylene-co-octene) elastomer, polyethylene-co-butene) elastomer,styrene-ethylene-butadiene-styrene elastomer, epichlorohydrin rubber,polyacrylic rubber, silicone rubber, fluorosilicone rubber,perfluoroelastomers, polyether block amides, chlorosulfonatedpolyethylene, ethylene-vinyl acetate, thermoplastic elastomer, proteinresilin, protein elastin, ethylene oxide-epichlorohydrin copolymer,polyurethane, urethane-urea copolymer, and combinations thereof.
 5. Thelithium metal secondary battery of claim 1, wherein said elastomercomprises from 0.01% to 50% of an electrically non-conductingreinforcement material dispersed therein, wherein said reinforcementmaterial is selected from the group consisting glass fiber, ceramicfiber, polymer fiber, glass particle, ceramic particle, polymerparticle, and combinations thereof.
 6. The lithium metal secondarybattery of claim 1, wherein said elastomer further contains from 0.1% to40% by weight of a lithium ion-conducting additive dispersed therein. 7.The lithium metal secondary battery of claim 6, wherein said lithiumion-conducting additive is selected from Li₂CO₃, Li₂O, Li₂C₂O₄, LiOH,LiX, ROCO₂Li, HCOLi, ROLi, (ROCO₂Li)₂, (CH₂OCO₂Li)₂, Li₂S, Li_(x)SO_(y),or a combination thereof, wherein X═F, Cl, I, or Br, R=a hydrocarbongroup, 0<x≤1 and 1≤y≤4.
 8. The lithium metal secondary battery of claim6, wherein said lithium ion-conducting additive contains a lithium saltselected from lithium perchlorate (LiClO₄), lithium hexafluorophosphate(LiPF₆), lithium borofluoride (LiBF₄), lithium hexafluoroarsenide(LiAsF₆), lithium trifluoro-methanesulfonate (LiCF₃SO₃),bis-trifluoromethyl sulfonylimide lithium (LiN(CF₃SO₂)₂), lithiumbis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF₂C₂O₄),lithium nitrate (LiNO₃), Li-fluoroalkyl-phosphate (LiPF₃(CF₂CF₃)₃),lithium bisperfluoro-ethylsulfonylimide (LiBETI), lithiumbis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide,lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-basedlithium salt, or a combination thereof.
 9. The lithium metal secondarybattery of claim 6, wherein said lithium ion-conducting additivecomprises a lithium ion-conducting polymer selected from the groupconsisting of poly(ethylene oxide) (PEO), polypropylene oxide (PPO),poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA),poly(vinylidene fluoride) (PVDF), poly bis-methoxyethoxyethoxide-phosphazene, polyvinyl chloride, polydimethylsiloxane,poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a sulfonatedderivative thereof, and combinations thereof.
 10. The lithium metalsecondary battery of claim 1, wherein said cathode active material isselected from an inorganic material, an organic material, a polymericmaterial, or a combination thereof, and said inorganic material does notinclude sulfur or alkali metal polysulfide.
 11. The lithium metalsecondary battery of claim 10, wherein said inorganic material isselected from a metal oxide, metal phosphate, metal silicide, metalselenide, transition metal sulfide, or a combination thereof.
 12. Thelithium metal secondary battery of claim 10, wherein said inorganicmaterial is selected from the group consisting of lithium cobalt oxide,lithium nickel oxide, lithium manganese oxide, lithium vanadium oxide,lithium-mixed metal oxide, lithium iron phosphate, lithium manganesephosphate, lithium vanadium phosphate, lithium mixed metal phosphate,lithium metal silicide, and combinations thereof.
 13. The lithium metalsecondary battery of claim 10, wherein said inorganic material isselected from a metal fluoride or metal chloride including the groupconsisting of CoF₃, MnF₃, FeF₃, VF₃, VOF₃, TiF₃, BiF₃, NiF₂, FeF₂, CuF₂,CuF, SnF₂, AgF, CuCl₂, FeCl₃, MnCl₂, and combinations thereof.
 14. Thelithium metal secondary battery of claim 10, wherein said inorganicmaterial is selected from a lithium transition metal silicate, denotedas Li₂MSiO₄ or Li₂Ma_(x)Mb_(y)SiO₄, wherein M and Ma are selected fromFe, Mn, Co, Ni, V, or VO; Mb is selected from Fe, Mn, Co, Ni, V, Ti, Al,B, Sn, or Bi; and x+y≤1.
 15. The lithium metal secondary battery ofclaim 10, wherein said inorganic material is selected from a transitionmetal dichalcogenide, a transition metal trichalcogenide, or acombination thereof.
 16. The lithium metal secondary battery of claim10, wherein said inorganic material is selected from the groupconsisting of TiS₂, TaS₂, MoS₂, NbSe₃, MnO₂, CoO₂, an iron oxide, avanadium oxide, and combinations thereof.
 17. The lithium metalsecondary battery of claim 11, wherein said metal oxide contains avanadium oxide selected from the group consisting of VO₂, Li_(x)VO₂,V₂O₅, Li_(x)V₂O₅, V₃O₈, Li_(x)V₃O₈, Li_(x)V₃O₇, V₄O₉, Li_(x)V₄O₉, V₆O₁₃,Li_(x)V₆O₁₃, their doped versions, their derivatives, and combinationsthereof, wherein 0.1<x<5.
 18. The lithium metal secondary battery ofclaim 11, wherein said metal oxide or metal phosphate is selected fromthe group consisting of layered compound LiMO₂, spinel compound LiM₂O₄,olivine compound LiMPO₄, silicate compound Li₂MSiO₄, tavorite compoundLiMPO₄F, borate compound LiMBO₃, and combinations thereof, wherein M isa transition metal or a mixture of multiple transition metals.
 19. Thelithium metal secondary battery of claim 10, wherein said inorganicmaterial is selected from the group consisting of (a) bismuth selenideor bismuth telluride, (b) transition metal dichalcogenide ortrichalcogenide, (c) sulfide, selenide, or telluride of niobium,zirconium, molybdenum, hafnium, tantalum, tungsten, titanium, cobalt,manganese, iron, nickel, or a transition metal; (d) boron nitride, and(e) combinations thereof.
 20. The lithium metal secondary battery ofclaim 10, wherein said organic material or polymeric material isselected from poly(anthraquinonyl sulfide) (PAQS), a lithium oxocarbon,3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA),poly(anthraquinonyl sulfide), pyrene-4,5,9,10-tetraone (PYT),polymer-bound PYT, quino(triazene), redox-active organic material,tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE),2,3,6,7,10,11-hexamethoxytriphenylene (HMTP), poly(5-amino-1,4-dyhydroxyanthraquinone) (PADAQ), phosphazene disulfide polymer ([(NPS₂)₃]n),lithiated 1,4,5,8-naphthalenetetraol formaldehyde polymer,hexaazatrinaphtylene (HATN), hexaazatriphenylene hexacarbonitrile(HAT(CN)₆), 5-benzylidene hydantoin, isatine lithium salt, pyromelliticdiimide lithium salt, tetrahydroxy-p-benzoquinone derivatives (THQLi₄),N,N′-diphenyl-2,3,5,6-tetraketopiperazine (PHP),N,N′-diallyl-2,3,5,6-tetraketopiperazine (AP),N,N′-dipropyl-2,3,5,6-tetraketopiperazine (PRP), a thioether polymer, aquinone compound, 1,4-benzoquinone, 5,7,12,14-pentacenetetrone (PT),5-amino-2,3-dihydro-1,4-dyhydroxy anthraquinone (ADDAQ),5-amino-1,4-dyhydroxy anthraquinone (ADAM), calixquinone, Li₄C₆O₆,Li₂C₆O₆, Li₆C₆O₆, or a combination thereof.
 21. The lithium metalsecondary battery of claim 20, wherein said thioether polymer isselected from the group consisting ofpoly[methanetetryl-tetra(thiomethylene)] (PMTTM),poly(2,4-dithiopentanylene) (PDTP), a polymer containingpoly(ethene-1,1,2,2-tetrathiol) (PETT) as a main-chain thioetherpolymers, a side-chain thioether polymer having a main-chain consistingof conjugating aromatic moieties, and having a thioether side chain as apendant, poly(2-phenyl-1,3-dithiolane) (PPDT),poly(1,4-di(1,3-dithiolan-2-yl)benzene) (PDDTB),poly(tetrahydrobenzodithiophene) (PTHBDT),poly[1,2,4,5-tetrakis(propylthio)benzene] (PTKPTB, andpoly[3,4(ethylenedithio)thiophene] (PEDTT).
 22. The lithium metalsecondary battery of claim 10, wherein said organic material contains aphthalocyanine compound selected from the group consisting of copperphthalocyanine, zinc phthalocyanine, tin phthalocyanine, ironphthalocyanine, lead phthalocyanine, nickel phthalocyanine, vanadylphthalocyanine, fluorochromium phthalocyanine, magnesium phthalocyanine,manganous phthalocyanine, dilithium phthalocyanine, aluminumphthalocyanine chloride, cadmium phthalocyanine, chlorogalliumphthalocyanine, cobalt phthalocyanine, silver phthalocyanine, ametal-free phthalocyanine, a chemical derivative thereof, andcombinations thereof.